The present studies are designed to determine the sequence of molecular events which link alpha2-adrenergic receptor occupancy to blockade of neurotransmitter secretion. Previous findings suggest that alpha2-adrenergic receptors provoke secretion from the human platelet by a pathway involving Na+/H+ exchange. Recently, we have observed alpha2-receptor activation of Na+/H+ exchange in a neuroblastoma x glioma hybrid cell line, NG108-15. Our proposed studies are intended to resolve the following questions: Do other receptor populations on NG108-15 cells that are linked to inhibition of adenylate cyclase, e.g. opiate and muscarinic receptors, also accelerate Na+/H+ exchange? Do GTP-binding proteins link receptor occupancy to activation of Na+/H+ exchange? Does differentation of NG108-15 cells to an electrically excitable state alter the properties of receptor-mediated Na+/H+ exchange? Does the alkalinization of NG108-15 cells that occurs as a consequence of Na+/H+ exchange cause an increase in outward K+ conductance, and associated hyperpolarization, thus providing a molecular basis for alpha2-receptor-mediated inhibition of neurotransmitter release? Does acceleration of Na+/H+ exchange activate a phospholipase A2 enzyme in NG108-15 cells, as it does in human platelets? If so, do arachidonic acid metabolites influence receptor-modulated neurotransmitter release? The present studies seek to provide novel insights into the sequence of events responsible for alpha2-adrenergic-receptor-induced changes in cellular responce by dissecting the pathway which leads from receptor occupancy to inhibition of neurotransmitter release. Perhaps more important is the probability that these studies will shed new light on the mechanism(s) by which all receptor populations linked to inhibition of adenylate cyclase modulate cellular response, since a large body of data suggests that decreases in cAMP levels, by themselves, are not sufficient to evoke changes in cell function.